Field of the Invention
The invention relates to a method for dampening a printing form for offset printing wherein a quantity of dampener applied to the printing form by transfer cylinders is controlled by adjusting the rate of rotation or rpm of one of the transfer cylinders in accordance with the rotary angle position of a channel or gap of a form cylinder carrying the printing form.
In German Utility Model DE 82 04 506 U1, an offset rotary printing press with a device for interrupting dampener feed is described, which is supposed to prevent local overdampening of a printing plate clamped to a plate cylinder formed with a channel or gap. When applying dampener to the printing plate with the aid of transfer cylinders, no dampener is dispensed from the transfer cylinder while there is a continuous transfer of dampener during the passage through or over the channel or gap. The quantity of dampener stored on the transfer cylinder is transferred to the next channel region of the printing plate, as viewed in the direction of rotation of the plate cylinder, due to which the aforementioned overdampening can occur. According to this German utility model, a control unit synchronized with the rotation of the plate cylinder is provided which, with the aid of control elements, causes the dampener feed to be interrupted in the channel region. Provided as a control element is a control cylinder, which causes a transfer cylinder, in a defined rotary angle region of the plate cylinder, to be lifted away from a dampener applicator cylinder periodically for each revolution of the plate cylinder. A disadvantage of this construction is that, by the engagement and disengagement of the transfer cylinder, mechanical impacts occur in the printing press which have a disadvantageous effect upon the printing quality.
In a modified form or variant of the foregoing embodiment embodiment disclosed in the German utility model, a blower device having an air flow which is directed towards a transfer cylinder for dampener is triggered likewise in a defined rotary angle region. The blown air causes a retention of the dampener located on the surface of the transfer cylinder. The effect of the flow of blown air is reduced if a high printing speed is realized. Moreover, with this construction, the danger arises that dampener will be randomly detached from the surface of the transfer cylinder by the air flow.
In the constructions according to the foregoing German Utility Model DE 82 04 506 U1, the control cylinder and the blower device, respectively, are controlled by a control cam. The control cam has a fixed control characteristic. There is no possibility of adapting it to the demand for dampener which is dependent upon the printed image.
An inking unit disclosed in the published German Patent Document DE 38 43 473 C2 operates so as to interrupt the transport of dampener during passage through the cylinder channel or gap; this is effected with a stripper or scraper which can be raised and lowered from a dampener applicator cylinder as a function of the rotary angle of the plate cylinder. Once again, mechanical impacts can impair the printing quality.
In German Patent 19 40 661 and in the published Japanese Patent Document JP 7-164 619 A, dampening units for offset printing presses are described wherein a transfer cylinder for dampener or dampening fluid is coupled with a motor. The rotary speed or rpm of the motor is adjustable as a function of the demand for dampener on a printing form. The transfer cylinder connected to the motor can thereby rotate at a different rotary speed or rpm from that of the adjacent cylinders which are in rolling contact with the transfer cylinder. The quantity of dampener transferred can be regulated by the slip existing between the transfer cylinders.
In these heretofore known constructions, only fluctuations in dampener on the surface of the printing form which occur over a relatively large number of printing operations are compensated for. The fluctuations in dampener occurring due to the cylinder channel or gap formed in the printing-form cylinder are not compensated for.